In an inkjet printer, a gray scale original image which is a continuous-tone image is halftoned (i.e., converted to a halftone image), and an image is represented by presence or absence of droplets of ink ejected onto a printing medium. An error diffusion method where quantization errors are distributed to peripheral pixels, a method where the original image is compared with a threshold matrix having threshold values with respect to every pixel to generate a halftone image and so on are known as halftoning methods.
Recently, since droplets ejected in the inkjet printer are miniaturized, representation of image with high resolution is achieved and sharp letters and lines can be printed. And also graininess of image is improved by miniaturization of droplet size so that a soft gradation can be obtained. On the other hand, since the weight of droplet of ink becomes lighter, flight of the droplet is unstable. Especially in single-pass printing, banding unevenness (mura) easily appears in a natural image and a tint image.
Japanese Patent Application Laid-Open No. 2006-123559 (Document 1) discloses an inkjet printer where input image data is multi-valued by the error diffusion method. In the inkjet printer, dot diameter of ink is switched between printing of image where good graininess (i.e., low graininess) is required even if resolution goes down and printing of image where high resolution is required. In the case where the error diffusion method is used, when small ink droplets are disposed in large numbers, graininess of image becomes better but substantial resolution goes down. In the inkjet printer, ink droplets having small ink dot diameter are used when required is an image having good graininess such as a portrait, and ink droplets having large ink dot diameter are used when required is an image having substantially high resolution such as a design drawing even if it has bad graininess (i.e., high graininess).
In a method of generating a printing plate image disclosed in Japanese Patent Application Laid-Open No. 07-264402 (Document 2), by applying a Laplacian filter to density data of an article image, filtered data showing local spatial frequencies is obtained. And by comparing the filtered data with a threshold value, either high resolution dots or low resolution dots are selected as printing dots to represent an image.
In Japanese Patent Application Laid-Open No. 61-285867 (Document 3), disclosed is a halftone image recording method where any one type of halftone dot screen is selected from a plurality of types of halftone dot screen having different shapes from one another, in accordance with a density band, hue band and coordinate band of an image.
In U.S. Pat. No. 6,532,082, disclosed is a halftone printing plate where a non-writing area is formed in halftone dot screen by a digital technique to improve printability.
In the case where the small ink droplets are frequently used in printing, banding unevenness easily appears in a range from middle to shadow of gray level in an image. On the other hand, in the case where the large droplets are frequently used, banding unevenness is suppressed but reproducibility of letters (text) and thin lines decreases.
In the method of generating a printing plate image in Document 2 and the multi-screening method in Document 3, measures to banding unevenness peculiar to the inkjet printer and switching of shape of halftone dot screen in order to improve quality of letters and thin lines are not considered. In Document 1, relationship between change of gray level in one image and dot diameter, and measures to banding unevenness are not considered also. Furthermore, in the case where the small droplets are frequently used and halftoning is performed with use of the error diffusion method as shown in Document 1, substantial resolution goes down.